The present invention relates to semiconductor structures, and particularly, to dense semiconductor fuse arrays having common cathodes.
Electrically operable fuses are utilized within the field of integrated circuit devices and processes for a number of purposes, including programming alterable circuit connections, or replacing defective circuit elements with redundant circuit elements. Electrically operable fuses are typically arranged in an array. Such fuse arrays typically comprise a number of fuses where each fuse can be individually selected and programmed. By activating a particular row and column of the fuse array, an individual fuse can be selected and programmed by driving sufficient current through the fuse, thereby causing it to heat up and eventually break. Once the fuse breaks, it can no longer pass current and is considered “programmed.”
FIG. 1 illustrates a conventional fuse 100. Fuse 100 comprises anode 110, cathode 120 and fuse link 130. Fuse 100 is typically formed from polysilicon. Cathode 110 can be approximately 1.38 um wide and 1.7 um long. Anode 120 can be approximately 0.68 um wide and 1.73 um long. Fuse link 130 can be approximately 0.12 um wide and 1.2 um long. Fuse link 130 is the component of fuse 100 that can be programmed. When a sufficient amount of current flows from anode 110 to cathode 120, fuse link 130 heats up and breaks. Adjacent fuses must be separated by a minimum amount of space (i.e. ground rule spacing) to ensure proper function, for example, a separation of 0.3 um between adjacent fuses can be sufficient.
FIG. 2 illustrates a conventional fuse array 200. Array 200 can comprise a plurality of fuses, each fuse being of the kind as previously described. As illustrated in FIG. 2, fuse array 200 is a 6×16 array (six rows by sixteen columns). Each fuse can be individually selected and programmed by activating the appropriate transistor within anode access circuit 220 and the appropriate transistor within cathode access circuit 230. For example, fuse 210 can be selected and programmed by activating anode access transistor 232 and cathode access transistor 234. For example, when cathode access transistor 234 provides ground potential to the cathode of fuse 210 and anode access transistor 232 provides some voltage potential above or below ground to the anode of fuse 210, current will flow through fuse 210, causing it to be programmed as previously described. Sense circuitry (not shown) can detect whether each individual fuse has been programmed.
Conventional fuse arrays like the one illustrated in FIG. 2 have several drawbacks. First, conventional fuse arrays are not efficiently area-optimized. In other words, conventional fuse arrays are not as densely arranged as possible, thereby negatively impacting the overall size of circuit designs that incorporate such arrays. For example, a conventional 6×16 fuse array of the kind illustrated in FIG. 2 having fuses of the kind illustrated in FIG. 1 has a total area of 883.2 um2 (9.2 um2 per fuse).
Additionally, conventional fuse arrays consisting of purely resistive fuse elements suffer from secondary currents that flow through a network of wires around the selected fuse. The secondary currents make the programming and sensing of individual fuses difficult. Therefore, a diodic fuse element that can limit the current in one direction is desired.
Also, conventional fuse arrays require two metal wiring levels for contacting the anode and cathode of each fuse. For example, as illustrated in FIG. 2, a first metal wiring layer 240 is required to form electrical connections between anode access circuit 220 and the anodes of each fuse (e.g. column lines) and a second metal wiring layer 250 is required to form electrical connections between cathode access circuit 230 and the cathodes of each fuse (e.g. row lines). The wiring levels must be insulated from one another to ensure proper function of the fuse array.
Therefore, a need exists for a dense fuse array having diodic fuse elements.